The present invention relates to a method of producing a cylinder in a two-cycle engine, especially a drive engine for a manually-guided implement such as, for example, a chain saw a brush cutter, a trimmer or the like.
Known two-cycle engines have a cylinder with a cylinder bore for receiving a piston. To reduce the wear of the movable parts, and to improve the operating characteristics, the inner side of the cylinder bore is provided with a coating, for example of chromium or nickel, with the surface of the coating forming a hardened piston gliding surface for the piston. Fresh gas from a fuel/air mixture is conveyed via at least one transfer channel into the inner chamber of the cylinder, whereby the transfer channel opens into the piston-gliding surface via an inlet window. The inlet window, relative to its geometrical design and its relative disposition in the piston-gliding surface, is arranged in such a way that the piston, by means of its reciprocating movement, opens or closes the inlet window at prescribed times. In this connection, the piston skirt of the piston cooperates with the inlet window as a slide control for that time interval during which fresh gas is conveyed into the inner chamber of the cylinder.
To achieve a high engine power, a high efficiency, and low exhaust gas values, it is necessary, in addition to having a precise disposition of the control times, to also have a precisely oriented guidance of flow through the inlet window that is as low in loss as possible. In this connection, the configuration of the window edges that surround the inlet window is of great significance. The fresh gas that flows in, in addition to filling the inner chamber of the cylinder, also has the task of pressing the exhaust gas, which resulted in a previous operating cycle, out of the inner chamber of the cylinder through an outlet channel. By means of a precisely oriented guidance of the fresh gas stream, the inner chamber of the cylinder should likewise be filled as much as possible with fresh gas, and the exhaust gas should be pressed out as completely as possible. In this connection, it is desired to keep scavenging losses due to fresh gas escaping together with the exhaust gas as low as possible. A sharp-edged configuration of the window edges is desired for a guidance of the flow that is as undisturbed as possible. Imprecisions in the configuration of the window edges can lead to an undesired turbulence or erroneous guidance of the fresh gas stream, as a result of which the operating characteristics of the internal combustion engine, and in particular the exhaust gas values that can be achieved, can be influenced in a disadvantageous manner.
Cylinders of the aforementioned type are conventionally produced from a light metal casting in which the transfer channels are cast. Introduced into the casting is a piston bore, the surface of which is machined for the subsequent application of a coating. In so doing, a sharp peripheral edge results at the inlet window. The surface of the cylinder bore is subsequently provided with a coating, for example of chromium or nickel, and is finished to size, especially by honing, for the piston that is later to be placed in the bore.
During the honing of the piston liner, or during deburring of the window edge, a portion of the coating can break away in the region of the window edge. The resulting imprecisions lead to a disadvantageous influencing of the guidance of the fresh gas stream, and hence to an impairment of the operating characteristics of the engine, and of the exhaust gas values that can be achieved.
It is therefore an object of the present invention to provide a method of producing a cylinder in a two-cycle engine by means of which a reliable and improved guidance of fresh air in the finished cylinder is ensured in conformity with the structurally prescribed design.